1. Field of the Invention
The present invention relates to a catalyst degradation determination device for an exhaust purification system. In particular, it relates to a catalyst degradation determination device for an exhaust purification system equipped a selective reduction catalyst that reduces NOx in exhaust under the presence of a reducing agent.
2. Related Art
Conventionally, as one exhaust purification system that purifies NOx in exhaust, a system has been proposed in which a selective reduction catalyst that selectively reduces NOx in the exhaust by way of a reducing agent is provided in an exhaust channel. For example, with an exhaust purification system of urea addition type, urea water is supplied from an upstream side of the selective reduction catalyst, ammonia is generated by thermal decomposition or hydrolysis of this urea water by the heat of the exhaust, and the NOx in the exhaust is selectively reduced by this ammonia. It should be noted that, in addition to such a system of urea addition type, for example, a system has also been proposed that generates ammonia by heating a compound of ammonia such as ammonia carbide, and directly adds this ammonia.
In an exhaust purification system equipped with a selective reduction catalyst, when degradation of the selective reduction catalyst progresses to a certain extent, it is necessary to replace this with a new catalyst in order to continuously maintain the exhaust purification performance thereof to be high. In the current situation, a degradation determination device that determines the degradation while the catalyst remains installed in the vehicle, i.e. during running of the vehicle, has been built into the current exhaust purification system in order to notify a time, which is an estimate for replacement of the selective reduction catalyst, to the driver or a service technician. Hereinafter, conventional technology for determining the degradation of a selective reduction catalyst will be explained with an exhaust purification system of urea addition type as an example.
FIG. 22 is a graph showing temperature characteristics of the NOx purification rate of a selective reduction catalyst. In FIG. 22, the NOx purification rate of a catalyst that is a new article is shown by the dashed-dotted line, and the NOx purification rate of a catalyst that has degraded from the new-article state is shown by the solid line.
For example, with the degradation determination device illustrated in Patent Document 1, degradation of a catalyst is determined based on detection of the NOx purification rate of the selective reduction catalyst, using the fact that the NOx purification rate will decline with the progression of degradation of the selective reduction catalyst, as shown in FIG. 22. More specifically, the NOx purification rate is estimated by estimating the NOx amount flowing into the selective reduction catalyst based on the operating state of the internal combustion engine, while detecting the NOx amount on a downstream side of the selective reduction catalyst, and then comparing this estimated value of the upstream side and detected value of the downstream side. Then, the degradation of the catalyst is determined based on the NOx purification rate thus estimated.
Incidentally, in addition to the ability to reduce NOx under the presence of ammonia, there is the ability to store ammonia generated in the selective reduction catalyst. Hereinafter, the ammonia amount stored in the selective reduction catalyst is referred to as storage amount, and the ammonia amount that can be stored in the selective reduction catalyst, i.e. maximum value of the storage amount, is referred to as storage capacity.
FIG. 23 is a graph showing temperature characteristics of the storage capacity of a selective reduction catalyst. In FIG. 23, the storage capacity of a catalyst that is a new article is shown by the dashed-dotted line, and the storage capacity of a catalyst that has degraded from the new-article state is shown by the solid line. It should be noted that, for comparison with the above FIG. 22, the same type of catalyst as the catalysts shown in FIG. 22 is used for this catalyst that is a new article and the degraded catalyst.
As shown in FIG. 23, the storage capacity of the selective reduction catalyst has a characteristic of declining over the entire temperature range when degradation of the catalyst progresses. In addition, as is evident from comparing FIG. 22 with FIG. 23, the magnitude of the change accompanying the progression of degradation becomes larger for the storage capacity than the NOx purification rate. This shows that the ammonia storage performance of the selective reduction catalyst is more suited as an index used in the determination of degradation than the NOx purification performance due to being able to improve the SN ratio.
Technology focusing on such ammonia storage performance of a selective reduction catalyst is illustrated in Patent Document 2 and Patent Document 3.
In Patent Document 2, a device is illustrated that, when the temperature of the selective reduction catalyst declines past a temperature region in which NOx can be purified, supplies urea water until ammonia slip occurs, converts a total amount of urea water supplied in this process to an ammonia adsorbed amount, and diagnoses the degradation of the selective reduction catalyst based on this ammonia adsorbed amount. Since ammonia slip occurs in response to the storage amount of the selective reduction catalyst having exceeded the storage capacity, it has been considered that the total amount of urea supplied in excess has a correlation to the storage capacity of the selective reduction catalyst; therefore, it may be said that the degradation of the catalyst is determined based on the storage capacity of the selective reduction catalyst with this device of Patent Document 2.
In addition, similarly to the device of Patent Document 2, a method is illustrated in Patent Document 3 of continuously supplying urea water until ammonia slip occurs at a predetermined reference operational point, and estimating the storage capacity of the selective reduction catalyst based on the total amount of urea water supplied in this process.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-138626    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2009-127496    Patent Document 3: Japanese Unexamined Patent Application Publication no. 2007-170383